To satisfy ever-increasing demands for data traffic, wireless communication systems have been developed to support higher data rates.
The tendency of the technological development of 4th Generation (4G) systems so far proposed is toward improvement of spectral frequency to increase data rate. However, it is difficult to satisfy soaring demands for data traffic simply through improved spectral efficiency.
One of techniques proposed to solve this problem is to use a very broad frequency band. However, it is very difficult to secure a broad frequency band in a cellular system operating at or below 5 GHz, which is a major example of wireless communication systems. Accordingly, it is necessary to secure a broadband frequency in a frequency band higher than that of the cellular system.
As a higher transmission frequency is used for wireless communication, propagation path loss increases. The resulting relative short propagation distance leads to reduction of service coverage. In this context, beamforming has been proposed as one of important techniques for mitigating propagation path loss and increasing a propagation distance.
A kind of beamforming, namely transmission beamforming is a scheme of steering a signal from each of a plurality of antennas in a specific direction. A set of such antennas is called an ‘antenna array’ and an antenna of the antenna array is called an ‘antenna element’.
In general, the propagation distance of a signal can be increased by transmission beamforming. Furthermore, transmission beamforming can reduce interference with other users because a signal is almost not transmitted in directions other than an intended direction.
A receiver may also perform reception beamforming using a reception antenna array. Reception beamforming can also increase the sensitivity of a signal received from a specific direction by focusing on wave reception from the specific direction and can eliminate interference by excluding signals from other directions from the received signal.
As a transmission frequency increases, the wavelength of a signal wave decreases. Accordingly, antennas may be configured at a half-wavelength interval in the antenna array. In this case, the antenna array may be formed with more antennas over the same area. That is, a communication system operating in a high frequency band can obtain a high antenna gain by beamforming, relative to beamforming in a low frequency band. Therefore, a communication system operating in a high frequency band is suitable for beamforming.
That's why the communication system operating in a high frequency band uses beamforming in order to mitigate great propagation path loss. Beamforming should apply to data and control signals without distinction.
Conventionally, beamforming in Institute of Electrical and Electronics Engineers (IEEE) 802.11ad involves Sector Level Sweep (SLS) and Beam Refinement Protocol (BRP).
The IEEE 802.11ad standard is a Wireless Local Area Network (WLAN)-based technology that provides a very small service area with a radius between 10 m and 20 m in a 60-GHz super-high frequency band. Especially to solve a propagation property problem encountered with a super-high frequency band, the IEEE 802.11ad standard recommends beamforming.
In the SLS scheme defined in IEEE 802.11ad, a STAtion (STA) which will perform beamforming transmits the same sector frame repeatedly in a plurality of directions. Then a peer STA receives each sector frame using a quasi-omni antenna and transmits a feedback regarding the direction that offers the best sensitivity. The STA may acquire information about the best direction from the peer STA and perform beamforming in the best direction.
In the BRP scheme defined in IEEE 802.11ad, after beamforming in the SLS scheme, the direction of a transmission and reception beam is subjected to fine adjustment in order to increase a transmission and reception beamforming gain.
Typically, after two STAs detect the best transmission beam by the SLS scheme, they use the BRP scheme to detect a reception beam most suitable for the transmission beam. In addition, a combination of the transmission and reception beam directions is finely adjusted.
Especially a communication system operating in a super-high frequency band (hereinafter, referred to as a ‘millimeter wave communication system’) will adopt beamforming to mitigate propagation path loss and increase a propagation distance, in view of wireless communication in the super-high frequency band.
To maximize an antenna gain by beamforming, the millimeter wave communication system should be able to select the best transmission/reception beam. For example, it may use the SLS and BRP schemes as proposed by IEEE 802.11ad to select the best transmission/reception beam.
For example, as many reference signals as the number of transmission-reception beam combinations are transmitted repeatedly to select the best transmission/reception beam. Each reference signal is transmitted and received in a specific transmission/reception beam and the best transmission-reception beam combination is selected by comparing the strengths of the received signals.
If narrower beams are used to obtain a higher antenna gain, the number of transmission/reception beams is increased. The resulting linear increase in the number of necessary reference signals increases overall system overhead and thus decreases total system capacity. Accordingly, there exists a pressing need for developing a method for minimizing the system overhead of reference signals required for selecting a transmission/reception beam, when beamforming is used in a millimeter wave communication system.